Breast cancer patients and their oncologists are often confronted with choices regarding radiation therapy. Randomized trials help to guide these increasingly complex decisions. Nevertheless, for certain clinical scenarios, randomized trial data are either incomplete, or are not tailored to adequately inform physicians and patients in decision-making. In the candidate's proposal, the decision to add radiation therapy will be studied using two approaches - in the setting of incomplete trial data at the health services level (Aim 1) and at the patient level using decision analysis to individualize outcomes (Aims 2-3). Aim 1 proposes to study decision-making at a population level using the SEER-Medicare database, by analyzing the factors that predict for the receipt of radiation therapy after mastectomy, The selection of patients who benefit from the addition of radiation in this setting is the subject of considerable controversy, due to the lack of complete data. In addition, the candidate will use decision analysis to bridge the gap between the large aggregates of patients studied in trials and the individual patient for two treatment decisions regarding radiation therapy. Aim 2 will use decision analysis to study the tradeoffs between breast-conserving therapy (lumpectomy and radiation) versus mastectomy for invasive cancers as they vary with patient age at diagnosis. Aim 3 is a decision analysis of the use of radiation therapy after breast-conserving surgery for small ductal carcinomas in situ versus surgery alone. For Aims 2 and 3, the candidate will construct models to synthesize the available data to estimate specific outcomes about the complex tradeoffs associated with these decisions. Model results will include both recurrence-free survival and time with an intact breast for different ages, so that a patient can use the results to inform decision-making by selecting the most important and relevant outcome, allowing for treatment decisions that are aligned with her individual preferences. By studying factors predicting for the decision to add radiation therapy after mastectomy in the population, and tailoring the presentation of data from trials to best serve the individual, completion of these projects will improve decision-making about radiation therapy for breast cancer patients. The candidate's career development plan includes additional coursework in epidemiology and risk and decision science. The proposed combined education and research program, in addition to strong mentorship, will provide the candidate with the skills and experience necessary for a career as an independent outcomes researcher in breast cancer.